M390

These fired bars show the progression of color from cone 4 (bottom) to cone 8 (porosity and fired shrinage are also indicated, cone 8 is over-fired).

M390 fires very dark and is the material of choice for dark burning functional ware. It contains more of our native Redstone material than any other body. It is one of several bodies that we process to 100 mesh particle size and is intended to produce ware with a clean, unblemished glaze surface.

M390 is very similar in composition to M350 and shares the very fine natural smooth character. It is however more plastic. Its darker color tends to limit it to specialized glaze effects. M390 contains 5 different clays, each of which is quite balanced on its own.

Process Properties

M390 has high plasticity and feels slick. It generates significant slip during throwing. Care and attention during drying are a must, especially with larger pieces, flat plates, shallow bowls, and sculptural ware. Make sure that the focus is on evenness of drying rather than speed. If sections of a piece dry faster, then either slow these sections down or slow down the entire piece. If you need to attach pieces (i.e. handles) use slip with a low water content and apply as much pressure and lateral movement as possible during the joining process.

Firing

M390 is a fairly high iron body and thus burns red to brown as it matures. At cone 3-4 it is a very warm toasty red. The red intensifies as it approaches cone 6 and turns brown by cone 7. We maintain the porosity at about 2.0% at cone 6 to hold onto the red color. We regard it as over-fired at cone 7 (although you might have success) but at cone 8 it will definitely be unstable (tending to bloat and warp). This instability occurs even though it has not reached zero porosity.

Transparent glazes will darken the color of the underlying body. If you fire to cone 7 keep in mind that accidental over firing could take it into the warping or bloating territory of cone 8. If you fire to cone 4, be aware that porosity is too high (and strength likely too low) for practical functional ware. Cone 6 is the best compromise of color, stability and strength.

Glazing

This mug is made from M390 and glazed with Alberta Slip rutile (outside) and GA6-A Alberta Slip base (inside).

This is G3806C clear on the outside and Alberta Slip GA6-A on the inside of an M390 mug. Most clear glazes will go cloudy, G3806C has enough melt fluidity to pass the bubbles. See below for more info on this glaze.

M390 is quite fine and produces a homogeneous fired surface for most glazes. Since it is a dark stoneware, the iron in the clay will bleed into glazes and colors and significantly stain or mute certain colors (compared to porcelain). On the other hand, this effect will enhance the appearance of earthtone and variegated glazes.

M390 is high in silica and will accept most typical cone 6 glazes without producing crazing. However, crazing is possible on M390 if a glaze is high in sodium (i.e. from soda feldspar or nepheline syenite) or is very low in silica or alumina (generally, glazes containing high feldspar and little kaolin or silica are usually a problem for any body). For functional ware we recommend you check glaze fit using a boiling water:ice water immersion test. Please contact Plainsman if you need help to adjust your glaze.

Transparent glazes: Although M390 fires to red, keep in mind that the color will darken considerably under a transparent glaze because the latter fluxes the surface of the clay. M390 is made from natural high iron stoneware clays and typical transparent glazes will often become milky (with clouds of micro bubbles). However under fluid melt transparents it looks alot better (our G3806C Panama Clear is a good example). An even better solution is to use an amber colored clear like GA6-A Alberta Slip base (the photo of the mug here shows it on the inside). Pay careful attention to the instructions of that page to keep it from crystallizing during cooling if you need the glossy surface.

Other Glaze Recipes

There are many glazes made from Alberta Slip and Ravenscrag Slip that you can use on M390 (see the cone 6 glaze page at each of these for more information). Consider joining the cone6pots group on the internet to learn more about many others (and contribute).